Prominent Figures in Chemistry

Evolution of the Model of the Atom

Democritus

• 460-370 BC

• Greek philosopher

• He believed that everything in the universe is made up of atoms and the void (empty space)

• the atoms is indivisible and cannot be broken down any further

Aristotle/Empedocles (Same person)

• 490-430 BC

• Greek philosopher

• He rejected the idea of the atom

• He proposed that everything was made up of 4 basic elements

  • earth

  • wind

  • fire

  • water

• People believed him for over 2000 years

John Dalton

• 1807

• He was a British school teacher

• Developed the billiard ball model

• The atom is uniform and a solid sphere

• No internal structure was known; indivisible

J.J Thompson

• 1897

• Conducted cathode ray tube experiments (a glass tube that is partially a vacuum)

• Discovered electrons

• Developed the plum pudding model/raisin bun model

• Model looks like a positive sphere with negative electrons dispersed throughout the atom

Ernest Rutherford

• 1911

• Conducted the famous gold foil experiment

• He shot alpha particles at a thin piece of gold foil; most particles went through but some deflected back

• Two conclusions

  • Most the atom is empty space

  • There is a positively charged nucleus surrounded by negative electrons

• Developed the nuclear model also known as the Rutherford model

Problems with the Rutherford model

• An accelerated charge should fall into nucelus

• Bohr revised Rutherford’s model to explain the existence of energy levels also called orbits

Niels Bohr

• 1913

• Electrons move in circular orbits within specific energy levels

• Developed the “Bohr” model also known as the “Bohr-Rutherford'“ mode

• Bohr model does not work for elements past 20 (Calcium)

Erwin Schrodinger

• 1926

• Quantum mechanics

  • Electrons can only exist in specified energy levels called orbitals and not orbits

  • Orbitals are regions or volumes of space where electrons are likely to be found

  • Orbitals take various shapes (s,p,d,f)

James Chadwick

• 1932

• Discovered neutrons inside the nucleus

• Revision of Rutherfords model

Paulie Exclusion Principle

• Only two electrons of opposite spin can occupy an orbital

Aufbau Principle

• Fill orbitals one at a time in order of increasing energy (according to the periodic table)

Hund’s Rule

• When filing electrons at the same sublevel (eg: p orbital), each orbital gets an electron before any get two electrons

Kirchoff

• Early 1800s

• Conducted the black body experiment

• A black body is a perfect black solid object that does not reflect any light and emits various forms of light (IR, visible, UV) as a result of its temperature

• Kirchoff took a black body and heated it at increasing temperatures and observed that light was emitted

Max Planck

• Max Planck interpreted Kirchoff’s black body experiment

• Max hypothesized that energy given off was not continuous but was emitted in small little bursts called quanta (bundle of energy)

Albert Einstein

• Albert Einstein used Planck’s theory to solve the mystery of the photoelectric effect

• Photoelectric effect - as light (quantum of energy) of a specific wavelength shines on a piece of metal, electrons are ejected